Queen Elizabeth II is leading commemorations to mark the 800th anniversary of the Magna Carta — but the human rights it is credited with enshrining are at the center of a modern political feud.

Prime Minister David Cameron joined the queen Monday at Runnymede, a riverside meadow near London where, in 1215, King John met disgruntled barons and agreed to a list of basic rights.

The Magna Carta — Latin for Great Charter — is considered the founding document of English law and civil liberties.

Cameron said it was modern Britons' duty to safeguard its "momentous achievement.''

But opponents accuse him of trying to undermine rights. Cameron's Conservative government wants to replace the Human Rights Act with a Bill of Rights, a move opponents fear could weaken key protections.